The invention relates to combustion furnaces, and, in particular, to a multi-stage combustor adapted for reduced emissions of NO.sub.X combustion products.
The market for industrial combustion equipment in the United States is shaped in large part by federal regulations governing air standards in urban areas, as mandated by the Clean Air Act (CAA), as amended. Industrial expansion can be limited in areas that do not meet National Ambient Air Quality Standards (NAAQS) for the emissions of certain combustion gases, such as NO.sub.2. New sources of NO.sub.X in non-attainment areas must use emission offsets and a tight level of control known as "lowest achievable emission rate (LAER)." A target NO.sub.X emission no greater than 9 ppmvd (parts per million by volume on a dry basis with 3% O.sub.2 in the emission) is usually established for new sources in non-attainment areas. The Clean Air Act also sets standards for ambient ozone in non-attainment areas and in other areas called "ozone transport regions," which meet the standard but into which ozone can migrate. New sources in some of the ozone non-attainment areas will be subject to the same LAER NO.sub.X target levels.
The 1990 amendments to the CAA affects smaller sources than previous regulations and consequently will impact industrial scale furnaces and boilers directly. The usual method to reduce NO.sub.X emissions to meet the LAER standards is to post-process exhaust gases employing selective catalytic reduction.